The present invention relates to a liquid crystal display comprising a liquid crystal display panel and a back light. More particularly, the present invention relates to the liquid crystal display which requires excellent color reappearance and visibility.
Recently, the liquid crystal display is used widely because of its advantages of lightness, thinness and low electric power usage, and replaces CRT which has been used.
FIG. 5 is a perspective view showing an arrangement of a general liquid crystal display. In FIG. 5, numeral 6 is a liquid crystal panel, numeral 7 is a back light unit. On the liquid crystal panel 6, there are provided a TFT element, lines and a TFT substrate 8 on which electrodes are formed, and a counter substrate 9 having a color filter on glass and aligned parallel and a liquid crystal 10 therebetween. An area 11 in the liquid crystal panel 6 is composed of a plurality of pixels, and each pixel comprising subpixel which transmits wavelength areas of red, green and blue. Light emitted by a light source 12 irradiates the whole surface of the liquid crystal panel 6 by the back light unit 7, and transmits through subpixel having a transmissivity corresponding to the transmissivity of a picture to be displayed, then observed by an observer.
Of characteristic of a displayed picture on the liquid crystal display, a color characteristic is generally expressed by a CIE chromaticity coordinates adapted by Commission Internationale de l'Eclairage. In the CIE chromaticity coordinates, a coordinate axis (x, y) showing two-dimensional chromaticity information is obtained from 3 stimulus values of X, Y and Z, derived from a value calculated by an integral calculus of a wave length range of visible light after multiplying a spectrum of an object to be targeted by a spectral sensitivity of human eyes. FIG. 6 shows the CIE chromaticity coordinates. In FIG. 6, a horseshoe shaped area 13 shows a range of visible light, and in this range, red is shown near by right bottom, green is upper part and blue is near left bottom. When a position Pr is a monochromatic red color in the color coordinate system, Pg is a monochromatic green color in the color coordinate system and Pb is a monochromatic blue color in the color coordinate system, a color reappearance area in which the liquid crystal display can display which is a domain represented the straight lines connecting three points, Pr-Pg-Pb, becomes a chromaticity in the color coordinate system as shown in an area 14.
The color characteristic of the liquid crystal display explained above is mainly determined by a combination of a characteristic of a light emitting spectrum of the back light and a characteristic of a transmit spectrum of the liquid crystal display. The characteristic of the transmit spectrum is usually determined by the characteristic of the transmit spectrum of a liquid crystal layer and the transmit spectrum of the color filter.
Numeral 15 in FIG. 7 shows a characteristic of the light emitting spectrum of a conventional back light of the liquid crystal display. With respect to the light source of the back light, a cold cathode tube is employed, and with respect to a fluorescent material, white-colored three wavelength fluorescent materials which is a mixture of a blue light emission, a green light emission and a red light emission, is employed. Therefore, the light emitting spectrum of the back light has three peaks, however, since the fluorescent material has the light emitting spectrum in the wavelength other than a main wavelength, the fluorescent material has peaks in areas between blue and green and in an area between green and red. Numerals 16, 17 and 18 in FIG. 7 show transmit spectrums of a conventional color filter. As shown in this figure, positions of the peak wavelength of the back light and positions of the transmit spectrum of a high transmit wavelength of the color filter generally coincide inside areas of blue, green and red. Numeral 19 in FIG. 8, the CIE chromaticity presents an input-output example of the color reappearance area. FIG. 7 also shows a color reappearance area 20 adapted by NTSC (National Television System Committee). In general, as a numerical value presenting the area of color reappearance of the display system, an area ratio for the color reappearance area 20 being adapted by NTCS is employed, and the value is called as Gamut. An example shown in 20, Gamut=50(%).
On the other hand, a Gamut value is about 72% for the conventional CRT, and its value is larger than the Gamut value of the conventional liquid crystal display. Recently, a demand for the liquid crystal display system becomes higher for TVs, and the color reappearance area for the liquid crystal display has been required to be high as the area of CRT. Therefore, in order to meet this requirement, the color filter has been improved. Numerals 21, 22 and 23 in FIG. 9 show the characteristic of the transmit spectrum of the color filter being developed for a purpose of obtaining a larger color reappearance area. In this spectrum characteristic, since layers of red area, green area and blue area become small, the color purity of each color, red, green and blue is improved, and possible display area of the color reappearance becomes larger. Numeral 24 in FIG. 8 shows the color reappearance area of the light emitting spectrum obtained from the color filter and a general back light. The Gamut value of this crystal display device is 72% and obtained the same level as the color reappearance area of a CRT.
Also in recent years, a requirement for the large color reappearance area for the liquid crystal display becomes even higher for a use of digital cameras and digital video cameras.
In accordance with Japanese Unexamined Patent Publication No. 97017/1997, through employing a white light source spectrum having a wavelength range which does not reach the peak at 470 nm to 510 nm and a wavelength range of 560 nm to 600 nm, the color purity becomes higher through eliminating unnecessary wavelength elements which locate near 490 nm for blue, near 490 nm and 580 nm for green.
In accordance with Japanese Unexamined Patent Publication No. 56812/2002, with respect to a light emitting source, an enlargement of the color reappearance area has been achieved by either eliminating or minimizing a side light emitting peak of a green fluorescent or reducing a band width of the light emitting spectrum of a blue fluorescent.
Furthermore, in accordance with Japanese Unexamined Patent Publication No. 277870/2002, within a range of wave length in which a spectral transmittance of a blue-colored filter layer is overlapped with that of green or a spectral transmittance of green-colored filter layer is overlapped with that of red, the enlargement of the color reappearance area has been achieved by not having a sub peak, which is different from a maximum peak in the light emitting spectrum of the fluorescent light source.
As mentioned above, in order to obtain the large color reappearance area for the conventional liquid crystal display, there has been primarily provided such a technique as to delete components other than a principal wavelength or set a right wavelength range of the light emitting spectrum for a fluorescent material of a fluorescent lamp. These methods emphasize an improvement of color purity of each color of red, green and blue through reducing the interference of each color, and does not mention about the hue itself.
However, as the liquid crystal display, the hue of each color of red, green and blue gives a large influence on clearness of a displayed picture, hence it is important. For example, as shown in above-mentioned Japanese Unexamined Patent Publication No. 97017/1997, when a proposed light source having the spectrum characteristic is employed, a position of the green chromaticity coordinates can be read as (0.28, 0.62) in FIG. 6, and (0.26, 0.64) in FIG. 8. However, these green colors are not genuine green colors, but within a range of so-called yellow green, and these hues are not sufficient to display a genuine green image.
Also, in accordance with Embodiment 1 of Japanese Unexamined Patent Publication No. 56812/2002, a position in the chromaticity coordinate system for a green color is (0.282, 0.645), and in accordance with Embodiment 2, a position in a chromaticity coordinates for a green color is (0.261, 0.642), therefore, both colors have yellow green hues. In addition, in Japanese Unexamined Patent Publication No. 277870/2002, there is no description about a specific wavelength of the light source spectrum, and there is no regulation of the hue of each color.
As mentioned above, the color characteristic of a displayed picture of the liquid crystal display is mainly obtained by the combination of the light emitting spectrum of the back light, the characteristic of the transmit spectrum of a liquid crystal layer and the characteristic of the liquid crystal layer of the color filter. However, as the prior art as mentioned above, either only the back light or the characteristics of the back light and the color filter is mentioned, and a characteristic of a spectrum of the display panel with a crystal layer is not mentioned. FIG. 10 shows the transmit spectrum of a general twist nematic (TN) type of the liquid crystal panel. As shown in FIG. 10, the characteristic of a spectrum is not flat for the wavelength range of the visible light, and it has a complicated characteristic. Therefore, in order to consider the color characteristic of the liquid crystal display, the characteristic of a transmit spectrum of the liquid crystal panel is essential.
In general, the liquid crystal display has a problem in a change of chromaticity and luminance when a direction of a viewing angle of a displayed picture is changed. As FIG. 11 shows, this occurs because the characteristic of the transmit spectrum on the crystal display changes widely by the viewing angle. An angle in FIG. 11 represents the viewing angle by left and right polar angles as a normal direction of 0° on the liquid crystal display panel. FIG. 12 shows a standardized spectrum from a front direction derived from the transmit spectrum of each angular direction of FIG. 11. On a low wavelength side, a change at a viewing angle of 60° is relatively more than 40% compared with a change at a front direction. Therefore, it is important to consider the characteristic of the liquid crystal panel in terms of having a good display picture when viewing angle is changed, in addition to an observation of the liquid crystal panel from the front direction.
An object of the present invention is to provide the liquid crystal display being capable of showing a genuine hue and chroma with a large picture of the color reappearance area, while controlling a variation of white trichromatic from a change of viewing angle.